1. Field of the Invention
The present invention relates to methods and devices for operating a danger alarm system, such as a fire alarm system, and in particular to a method and apparatus for increasing the response sensitivity of the alarm units in the alarm system while also increasing the interference resistance of the alarm units.
2. Description of the Prior Art
Automatic alarm systems such as fire alarm systems generally consist of a plurality of alarm units connected to a central station which each continuously emit alarm measured values which are cyclically sampled and evaluated at the central station. The alarm units in alarm systems such as fire alarm systems monitor a number of parameters such as smoke density, temperature, and radiation which are each weighted and evaluated in order to trigger an alarm signal. Each alarm unit has a characteristic interference resistance which is the ability of an alarm unit to "ignore" the various danger parameters until those parameters individually and/or in combination reach danger levels thus in theory preventing false alarms. Each alarm unit may, for example, contain a threshold circuit dedicated to each monitored parameter which emits an alarm signal to the central station whenever the threshold is exceeded. In order to increase the interference resistance, and thus further minimize the possibility of false alarms, the central station may contain timing circuits which indicate an alarm only when the threshold of one or more threshold circuits has been exceeded for a specified length of time. Such absolute threshold circuits may be employed in combination with threshold circuits which monitor the change over a period of time of a selected parameter, with a rate of change above a selected rate triggering an alarm. A competing design goal in alarm systems is that of designing an alarm system with a high response sensitivity, which is the ability of the alarm system to trigger an alarm signal every time true alarm conditions exist. The interference resistance of an alarm unit cannot be made so high as to significantly decrease the response sensitivity, otherwise true alarm conditions may fail to trigger an alarm signal.
A problem affecting both the interference resistance the the response sensitivity of alarm units is that of changing electronic component values associated with the electronic components comprising an alarm unit due to aging, dirt, humidity and the like. An evaluation threshold which may be set at the time of installation of an alarm unit may be satisfactory at the time of installation but, as a result of changing component values over a period of time, may no longer be acceptable and may trigger false alarms or cause true alarm conditions to fail to trigger an alarm.